Tropical Weather Discussion

By: NWS
By: NWS

TROPICAL STORM COLIN DISCUSSION NUMBER 13
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL042010
1100 PM AST FRI AUG 06 2010

COLIN IS A SHEARED CYCLONE WITH INCREASING CONVECTION NEAR AND TO
THE EAST OF THE CENTER AT THIS TIME. THIS COULD GIVE AN IMPRESSION
OF A STRONGER CYCLONE BUT FLUCTUATIONS IN CONVECTION ARE TYPICAL IN
SHEARED SYSTEMS. THE STRONG UPPER-LEVEL WINDS CURRENTLY OVER THE
CYCLONE WILL PROBABLY SEPARATE THE CENTER FROM THE THUNDERSTORM
ACTIVITY AGAIN SOON. THEREFORE...THE INITIAL INTENSITY IS KEPT AT
40 KNOTS. ANOTHER RECONNAISSANCE PLANE WILL CHECK COLIN IN SEVERAL
HOURS. COLIN HAS A SMALL WINDOW OF OPPORTUNITY TO GATHER SOME
STRENGTH WITHIN THE NEXT DAY OR SO...IF THE SHEAR RELAXES A BIT AS
DIAGNOSED BY THE SHIPS MODEL. IN FACT...IF THE CURRENT INCREASE IN
CONVECTION CONTINUES OR PERSIST IT COULD MEAN THAT THE SHEAR IS
ALREADY RELAXING AND SLIGHT INTENSIFICATION IS ABOUT TO BEGIN. THE
OFFICIAL FORECAST ASSUMES THAT THE SHEAR WILL RELAX A BIT AND CALLS
FOR A SMALL INCREASE IN INTENSITY. BY 96 HOURS...COLIN SHOULD BE
EXTRATROPICAL AND THEN BECOME ABSORBED BY A LARGER CYCLONE. THE
INTENSIFICATION SHOWN IN BOTH THE GFDL AND HWRF MODELS OCCURS LATE
IN THE FORECAST PERIOD. THIS DOES NOT SEEM TO BE REALISTIC BECAUSE
BY THEN...THE CYCLONE IS EXPECTED TO BE REACHING COOLER WATERS AND
BECOMING EXTRATROPICAL.

THE CENTER IS CLEARLY OBSERVED ON THE WESTERN EDGE OF THE CONVECTION
AND ONE CAN OBTAIN A GOOD ESTIMATE OF THE INITIAL MOTION...WHICH IS
TOWARD THE NORTH OR 010 DEGREES AT 8 KNOTS. COLIN IS ALREADY ON THE
WESTERN EDGE OF THE SUBTROPICAL RIDGE AND IS NEARING THE
MID-LATITUDE WESTERLIES. THEREFORE...THE CYCLONE SHOULD GRADUALLY
TURN MORE TO THE NORTH-NORTHEAST WITH SOME INCREASE IN FORWARD
SPEED. THERE IS GOOD CONFIDENCE IN THE FORECAST BECAUSE THE TRACK
GUIDANCE IS PACKED AND THE OFFICIAL FORECAST IS IN THE MIDDLE OF
THE ENVELOPE. THE ONLY DIFFERENCE IS IN THE FORWARD SPEED SHOWN BY
VARIOUS MODELS...BUT THIS MOSTLY OCCUR AT THE LONG RANGE AS THE
SYSTEM BEGINS THE TRANSITION TO AN EXTRATROPICAL LOW.


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